1. Field of the Invention
The present invention relates to a positive electrode active material of a positive electrode for non-aqueous electrolyte secondary battery, process for preparing the same, and a positive electrode for non-aqueous electrolyte secondary battery using such a positive electrode active material for non-aqueous electrolyte secondary battery and a non-aqueous electrolyte secondary battery. More particularly, a feature of the invention is an improvement in a positive electrode active material of a positive electrode for a non-aqueous electrolyte secondary battery in a non-aqueous electrolyte secondary battery for the purpose of improving charge/discharge cycle performances at high voltage and enhancing charge/discharge efficiency after preservation at high temperature.
2. Description of the Related Art
In recent years, mobile information terminal such as mobile phone, note personal computer and PDA has been rapidly developed to be more compact as well as lighter in weight. As a result, demands for higher capacity in a battery as a driving power source used for such a mobile information terminal have been increasing.
In order to meet such a demand, in recent years, as one type of new secondary batteries featuring high powers and high energy densities, a non-aqueous electrolyte secondary battery is in wide use, which employs a non-aqueous electrolyte and which is adapted for charging/discharging by way of transfer of lithium ions between a positive electrode and a negative electrode.
Generally, the positive electrode active material used for the positive electrode of such a non-aqueous electrolyte secondary battery is lithium cobalt oxide LiCoO2, lithium manganese oxide LiMn2O4 having a spinel structure, lithium composite oxide of cobalt-nickel-manganese, lithium composite oxide of aluminum-nickel-manganese, lithium composite oxide of aluminum-nickel-cobalt, and the like. The negative electrode active material of the negative electrode used includes carbon materials such as graphite, materials to be alloyed with lithium such as Si and Sn, and the like.
In recent years, because of advancement of amusement function such as moving image reproduction and game function by mobile information terminal, electric power consumption tends to rise steadily. As a result, demands for further higher capacity in a battery have been increasing.
In order to increase capacity further, it is thought to raise charging termination voltage.
For example, a non-aqueous electrolyte secondary battery which uses lithium cobalt oxide having high capacity is generally charged until voltage becomes about 4.3 V with respect to lithium metal, which means the capacity of lithium cobalt oxide is utilized only at about 160 mAh/g, although theoretical capacity of lithium cobalt oxide is about 273 mAh/g.
On the other hand, when the non-aqueous electrolyte secondary battery which uses lithium cobalt oxide having high capacity is charged until voltage becomes 4.5 V with respect to lithium metal, lithium cobalt oxide is utilized to the degree of about 180 mAh/g and a non-aqueous electrolyte secondary battery with higher capacity can be obtained.
However, if charging voltage of the non-aqueous electrolyte secondary battery is raised as described above, oxidizing force of the positive electrode active material such as lithium cobalt oxide becomes strong. As a result, speed of decomposition of the non-aqueous electrolyte is accelerated, stability of crystal structure of the positive electrode active material in which lithium is removed is deteriorated, and elements such as cobalt are eluted from the positive electrode active material.
Then, decomposition products of the non-aqueous electrolyte and the elements such as cobalt eluted from the positive electrode active material are moved to the negative electrode, forming a film on the surface of the negative electrode. By such a condition, charging-acceptance of the negative electrode is lowered and lithium metal is deposited on the surface of the negative electrode. As a result, cycle characteristics and preservation characteristics of the non-aqueous electrolyte secondary battery are greatly deteriorated. Especially, under the strict circumstances of leaving the non-aqueous electrolyte secondary battery as it is at high temperature of 50° C. or more, the non-aqueous electrolyte secondary battery is greatly deteriorated.
In recent years, in order to prevent elements such as cobalt from eluting from the positive electrode active material, it has been proposed to use a positive electrode active material produced by coating the surface thereof with metal oxide such as Mg and Al by sol-gel method using metal alkoxide such as Mg and Al. (See, for example, Patent Document 1, Japanese Published Unexamined Patent Application No. 11-317230.) Further, it has been proposed to use a positive electrode active material particle produced by coating the surface thereof with metal such as Mg and Al by mechanical method using ball mill or the like. (See, for example, Patent Document 2, Japanese Published Unexamined Patent Application No. 2006-173099).
However, as suggested in Patent Document 1, since metal alkoxide, such as Mg and Al, is expensive, the method using such metal alkoxide leads to a problem of cost increase.
Further, unfortunately, in both methods suggested in these Patent Documents, it has been difficult that metal alkoxide such as Mg and Al is adhered being uniformly distributed on the surface of the positive electrode active material. As a result, in the case of raising the charging voltage, there still remains a problem of deterioration in cycle characteristics and preservation characteristics of the non-aqueous electrolyte secondary battery which is resulting from decomposition of the non-aqueous electrolyte and the elution of elements such as cobalt from the positive electrode active material.